The present invention relates to structures, methods and programs for controlling a support structure in a lithographic apparatus.
A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, a patterning device, also referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. Such a pattern can be transferred onto a target portion (e.g., comprising a part of, one, or several dies) on a substrate (e.g., a silicon wafer). The pattern is typically transferred by imaging the pattern onto a layer of radiation-sensitive material (e.g. a “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called “steppers”, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called “scanners”, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One of the challenging requirements in micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of structures with respect to each other. For example, sub-100 nm lithography demands substrate-positioning and mask-positioning stages with dynamic accuracy and matching between machines in the order of 1 nm in 6 degrees of freedom (DOF), at velocities up to 3 m/s.
A conventional approach for achieving such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table), onto which a fine positioning module is cascaded. The coarse positioning module has a micrometer accuracy. The fine positioning module is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers. The coarse positioning module covers a large working range, while the fine positioning module only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning modules include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in six degrees of freedom (DOF), large-range motions are rarely required for more than two DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy required for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schäffel et al. “Integrated electro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting, California, USA, 1996, p. 456-461. Similar encoders are also available commercially, e.g., position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH of Traunreut, Germany.
Position measurement for mask and substrate tables at the end of the fine positioning module, on the other hand, has to be performed in 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
As an alternative for interferometers, optical encoders can be used, optionally in combination with interferometers. Such optical encoders are, for instance, disclosed in U.S. Patent Application Publication No. 2004/0263846 A1 titled “Lithographic Apparatus, Device Manufacturing Method, and Device Manufacturing thereby” of Kwan, which is hereby incorporated herein by reference. In Kwan, optical encoders use a grid pattern on one or more grid plates to determine their position with respect to the grid pattern. The optical encoders are mounted on the substrate table, while the grid plate is mounted on a frame of the lithographic apparatus. As a result, the location of the substrate table is with respect to the grid plate can be inferred.
With continued efforts to image ever smaller patterns to create devices with higher component densities, while increasing the number of patterns manufactured per unit time, opto-electro-mechanical processes within the lithographic apparatus need to be performed faster leading to vibrations of the substrate table from high acceleration and decelerations. Such vibrations make the job of alignment more difficult. Even though an alignment system and the grid plate may be coupled to the same frame within the lithographic apparatus, their relative position stability becomes insufficient to perform alignment of a substrate with respect to the substrate table at desired levels of accuracy.